Recently, aluminum nitride (AlN) has become a very important material in industrial applications due to its unique physical properties, such as high thermal conductivity close to that of metals and more than 10 times as much as that of alumina; high electrical resistivity and low thermal expansion coefficient comparable to silicon and silicon carbide; a high electrical resistivity; good thermal-shock resistance; mechanical strength comparable to alumina ceramics; and good corrosion resistance. Aluminum nitride has been popularly considered for use in many commercial applications, such as substrates for electronic components; integrated circuit packaging materials; heat dissipation materials; and vessels for containing or processing molten metals and salts.
Aluminum nitride are generally manufactured via the method of, carbothermally reducing and nitriding alumina; directly reacting aluminum metal with nitrogen; reacting aluminum chloride and ammonia in a vapor phase; and self-propagating high temperature synthesis (SHS), i.e. combustion synthesis. The first three methods mentioned above and their modified processes all have the common drawbacks of taking relatively much time; consuming relatively much energy; having relatively low conversion rate, etc. In comparison with those methods, the method of combustion synthesis is newly developed to produce aluminum nitride, and has the advantages including fast reaction rate; less energy consumption; simple manufacturing steps suitable for use in mass production; and higher conversion rate. The method of combustion synthesis basically uses a spontaneous chemical reaction initiated by igniting and rapidly propagating a combustion wave through a reactant admixture to produce aluminum nitride. The factors of successfully producing aluminum nitride by combustion synthesis include providing sufficient nitrogen fully penetrating the reactant admixture; preventing agglomeration of aluminum nitride produced, thus benefiting the subsequent grinding process; and achieving a complete reaction. Additionally, controlling the temperature of nitrogen gas; recycling the nitrogen gas; and cooling the reaction chamber wall will further enhance the quality and productivity of preparing aluminum nitride; prolong the life of reaction chamber; and reduce the production cost.
Several prior arts for preparing aluminum nitride by the combustion synthesis method are described hereinafter:
(1) In Japan Patent No. 63-274605, aluminum powder and sodium nitride (NaN3) powder or, such as potassium nitride (KN3), barium nitride (Ba3 N2), etc., are well mixed in accordance with an appropriate ratio, and thereafter the mixture is placed in a refractory container. After an igniting agent is placed on the mixture, the refractory container is placed into an electric oven that will be filled with nitrogen gas at a pressure less than or equal to 10 kg/cm2. The reactants are preheated in the electric oven, and then ignited by an electric wire to perform the combustion reaction so as to form an aluminum nitride powder.
(2) Uda et al. (“Preparation of Mixed Ultrafine (Al+AlN) Powders and Their Nitridation”, Physical Chemistry of Powder Metals Production and Processing, The Minerals, Metals & Materials Society, 1989.) disclosed the preparation of AlN sintered compacts using a simple furnace, wherein the compact of mixed ultra-fine (Al+AlN) powder is placed in a cold furnace and heated in a nitrogen atmosphere. When the temperature reached about 870 K, the combustion of the compact accompanied by an intense emission of light occurs, and the temperature of the compact rises from about 870 K to about 1700 K in a few seconds, thereby obtaining a hard and porous sintered compact of AlN.
(3) Clark et al. (“Combustion Synthesis Using Microwave Energy”, Ceram. Eng. Sci Proc. 11[9–10], pp. 1729–1742, 1990.) disclosed a combustion synthesis process of pouring Al powder into a raised silica crucible; then placing the crucible in a microwave for 5 minutes with consistent nitrogen gas, so as to purge the air; and thereafter actuating microwave source to initiate SHS.
(4) Miyamoto (“New Ceramic Processing Approaches Using Combustion Synthesis Under Gas Pressure”, Ceramic Bulletin, Vol. 69, No. 4, pp. 686–690, 1990.) disclosed a method for preparing AlN by combustion synthesis, wherein Al powder is placed in a porous graphic crucible, and then the crucible is placed in a reactor in which nitrogen gas is filled to the pressure range of 1–10 Mpa.
(5) In the paper of Long et al. (“Aluminum Nitride, a Refractory for Aluminum for 2000° C.”, Journal of American Ceramic Society, Vol. 42, No. 2, pp53–59, Feb. 1, 1959), it is stated that Mellor (“Comprehensive Treatise On Inorganic and Theoretical Chemistry”, Vol. VIII, Nitrogen and Phosphorus. Longmans, Green and Co., New York, 1928.) reported that Brieglib and Geuther, in 1862, produced aluminum nitride by heating aluminum turnings in an atmosphere of nitrogen, wherein the reaction proceeds to about 700° C. Mellor also reports that Zengheis disclosed the formation of aluminum nitride, wherein aluminum was burned in oxygen, and nitrogen is substituted with oxygen while the metal was still burning.
The conventional combustion synthesis methods for preparing aluminum nitride can be roughly divided in two categories: forming a compact by molding the reactant admixture; and filling the reactants into a refractory container, such as a graphite or ceramic crucible, wherein the former needs to preprocess the reactant admixture into a compact before reaction, resulting in higher operation cost and complicated operation steps, and the latter has difficulty in achieving the preparation of aluminum nitride of good quality and production. Moreover, none of the conventional technologies can fully achieve the aforementioned factors of successfully producing aluminum nitride powder by combustion synthesis.
Hence, there is an urgent need to develop a method and an apparatus for preparing aluminum nitride for satisfactorily meeting the aforementioned factors of successfully producing aluminum nitride powder by combustion synthesis, and overcoming the shortcomings of the conventional technologies.